Background Medium Research Articles

Hybrid Metal-Dielectric Metasurfaces for Refractive Index Sensing.

Hybrid metal-dielectric nanostructures have recently gained prominence because they combine strong field enhancement of plasmonic metals and the several low-loss radiation channels of dielectric resonators, which are qualities pertaining to the best of both worlds. In this work, an array of such hybrid nanoantennas is successfully fabricated over a large area and utilized for bulk refractive index sensing with a sensitivity of 208 nm/RIU. Each nanoantenna combines a Si cylinder with an Al disk, separated by a SiO2 spacer. Its optical response is analyzed in detail using the multipoles supported by its subparts and their mutual coupling. The nanoantenna is further modified experimentally with an undercut in the SiO2 region to increase the interaction of the electric field with the background medium, which augments the sensitivity to 245 nm/RIU. A detailed multipole analysis of the hybrid nanoantenna supports our experimental findings.

Study on the factors of large-scale space wave absorption of MWCNTs/Fe3O4 nanocomposite particles

With the application of nanoparticles in the field of microwave absorption more and more widely, so it is of great practical value to apply them in large-scale space, and the research on the influencing factors of nanoparticles in large-scale space becomes the key. Considering the diversity of nanocomposites, MWCNTs and Fe3O4, which are easy to prepare, are selected to composite. First, MWCNTs/Fe3O4 nanocomposites are prepared by ball milling and characterized by XRD, TEM and network vector analyzer. Then, the electromagnetic parameters are equivalent by using the strong disturbance equivalent theory, and the absorption loss in the frequency range of 2–18 GHz is calculated by numerical simulation. Finally, the influence of the volume ratio of particles (fs), background medium (eb) and thickness of absorbing layer (d) on electromagnetic wave absorption loss in space is analyzed by orthogonal experiment, and the absorbing mechanism is analyzed.

Design of highly-efficient acoustic waveguide couplers using impedance-tunable transformation acoustics

In this paper, the theory of impedance-tunable transformation acoustics in the geometric-acoustics limit is proposed to design efficient two-dimensional acoustic waveguide couplers. By choosing suitable impedance functions in the original space, impedance matching between the transformation medium and the background medium becomes possible, and the reflection at the boundary is reduced. The theory can be used to enable efficient acoustic coupling between waveguides of different sizes and different embedded media. By selecting an appropriate impedance function and a tunable acoustic refractive index, the transformed medium in the coupler can become a simplified parameter medium, for which the bulk modulus is a constant. This makes the experiment substantially easier. The problem of a reduced coupling-efficiency at low frequencies (a deviation from the geometric acoustic approximation) can be mitigated by selecting a large acoustic refractive index. Our two-dimensional numerical simulations indicate that this theoretical design works very well. The method can be extended to other transformation acoustic designs including three-dimensional cases.

Accounting for the Heisenberg and Pauli principles in the kinetic approach to neutrino oscillations

While oscillations of solar neutrinos are usually studied using the single-particle quantum-mechanical approach, flavor conversions of supernovae neutrinos are typically analyzed using the kinetic equation for the matrix of densities due to the necessity of including also the scattering processes. Using the Wigner formulation of quantum mechanics we show the equivalence of the quantum-mechanical and kinetic approaches in the limit of collision-less neutrino propagation (in a background medium). Based on this observation we also argue that solutions of the kinetic equation account for the Heisenberg uncertainty principle and the related effect of wave packet separation (for single neutrinos), as well as the Pauli exclusion principle, if the initial conditions are consistent with these fundamental quantum principles. Such initial conditions can be constructed e.g. by identifying the matrix of densities with the (reduced) single-particle Wigner function computed using initial conditions for the neutrino wave function. Hence the neutrino momentum uncertainty is an integral part of the initial conditions for the matrix of densities, that may have an impact on the phenomenology of supernovae neutrinos via the effect of wave packet separation.

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Abstract Large reservoirs of cold (∼104 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmic-ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.

Steady-state temperature distribution under near-field radiative heat transfer inside a linear chain of polaritonic nanoparticles

Thermal transport mediated by near-field interactions is superdiffusive in an ordered network of nanoparticles, such as linear chains. Based on such nanostructures, here we study a classical thermal-nonequilibrium problem, that is, the steady-state temperature distribution inside the chain of equally spaced polaritonic nanoparticles. The existence of the linear temperature profile and anomalous temperature variations in the inner region and near the boundary of a dense chain embedded in vacuum, respectively, is demonstrated by analyzing the effective radiative thermal conductivity within the system. Such temperature distributions are closely relevant to the strength of propagating surface modes sensitive to the chain density and dielectric material. We also demonstrate that, the presence of a homogeneous nonabsorbing background medium renders the steady-state temperature profile of the dense chain nonlinear, as well as enhancing drastically the long-distance heat exchange by two orders of magnitude relative to the vacuum case. This enhancement is a result of the high density of propagating waves in the medium, and is further confirmed by analyzing the energy density distribution over the chain. Furthermore, in a complex system consisting of many parallel chains, we show that, in steady state, temperatures of the thermal drive chain are allowed to tailor by its neighboring chains due to many-body interactions. This work opens ways to qualitatively assess and manipulate the stationary temperature field in advanced nanostructures, such as topological plasmonic chains and nanoparticle arrays.

Reflection and transmission coefficient approximations for P, S1 and S2 waves in triclinic media

SUMMARY The conventional assumptions, in most published approximations of the reflection and transmission (R/T) coefficients of plane waves at a plane interface between two anisotropic half-spaces, confine their applications to weakly anisotropic and/or weak contrast models. We consider the horizontal interface enclosed by two triclinic half-spaces to approximate the R/T coefficients normalized by the vertical energy flux. The homogeneous background medium can be anisotropic with arbitrary symmetry to better simulate the strongly anisotropic media. The second-order approximations are proposed to accommodate the strong contrast interface. We also consider an isotropic background medium under the weak anisotropy assumption. The obtained approximations can be applied to P, S1 and S2 waves, except for the transmission coefficients between the S1 and S2 waves. The S-wave transmission coefficients are insensitive to the model parameter contrasts and predominately rely on the S-wave polarization directions in the half-spaces above and below the interface. The proposed approximations are tested numerically.

An Improved Subspace-Regularized DBIM-MLGFIM Method for Three-Dimensional Inverse Scattering Problems

The repetitive computation of the forward problem in distorted Born iterative method (DBIM)-type inversion method is time-consuming and memory-costly, especially in dealing with 3-D inverse scattering problem. An improved subspace-regularized DBIM is proposed for the efficient solution of the 3-D inverse problem, which introduces the multilevel Green’s function interpolation method (MLGFIM) to accelerate the computation of the forward problem. In order to stabilize the inversion, an improved subspace-regularized DBIM is used with one new subspace-regularized induced current term in the cost function in virtue of truncated singular value decomposition (TSVD). To further reduce the computation burden, the incident field induced with updated inhomogeneous background medium is used approximately as the total field in the cost function, which saves one forward solver calculation in each iteration. Besides, the resource-intensive multiplication of inhomogeneous Green’s function can be avoided, which further improves the efficiency. Three numerical and two Fresnel experimental examples are given to demonstrate both the efficiency of the forward solver and the accuracy of the inversion algorithm.

Simultaneous Effects of Temperature and Pressure on the Entropy and the Specific Heat of a Three-Dimensional Quantum Wire: Tsallis Formalism

In this work, the finite-difference time domain (FDTD) has been employed to calculate the energy levels and wave functions of a three-dimensional (3D) cylindrical quantum wire. The inside of the wire is at zero potential, and the background medium is 4.6 eV. This is a true 3D procedure based on a direct implementation of the time-dependent Schrodinger equation. Here, the dependence on the pressure and the temperature of the electronic effective mass is used. We study the effects temperature and pressure simultaneously on the entropy and the specific heat of the system using the Tsallis formalism. The results show that (i) the specific heat obtained by Tsallis has a peak structure. (ii) The entropy has almost the same values at very low temperatures with different pressures. (iii) The peak value of the specific heat with enhancing the pressure shifts toward lower temperatures. (iv) The peak value of the specific heat and its position depend on the value of the entropic index q.

Contraction Integral Equation for Modeling of EM Scattering by 3-D Bodies Buried in Multilayered Anisotropic Medium

Integral equation (IE) is regarded as an efficient method for 3-D modeling of electromagnetic (EM) responses. The main contribution of this article is to develop a contraction integral equation (CIE) algorithm with a high performance for modeling of EM scattering by 3-D bodies buried in multilayered anisotropic medium. The introducing of contraction operator can ensure that the discretized IE will be iteratively convergent in large conductivity contrast between abnormal bodies and background medium. Moreover, the contraction operator can also accelerate the iteration convergence of the IE. During the discretization process of CIE, direct waves are extracted from dyadic Green's functions (DGFs) of multilayered medium, and the singular integrals of direct waves are eliminated using equivalent sphere and ellipsoid methods. By splitting DGF into convolution terms and correlation terms, 3-D FFT can be used to accelerate the product evaluation of DGFs integral and scattering current encountered in each iteration. Validity of the proposed CIE is verified by comparing with the existing finite volume method. Modeling results show that the conventional IE will be iteratively divergent when the conductivity contrast between bodies and background is large, but the CIE is always iteratively convergent even under very large conductivity contrast.

DETECTION OF NEAR-SURFACE TRACES FROM UNDERGROUND NUCLEAR EXPLOSIONS AT SEMIPALATINSK TEST SITE USING ACTIVE SEISMIC METHOD

In 1997, Institute of Geophysical Research NNC RK conducted seismic study of the Balapan site at Semipalatinsk Test Site, within the borders of which 14 underground nuclear explosions were carried out earlier. One of the tasks of the study was to develop a detection methodology in the upper part of the traces section from underground nuclear explosions as target objects during the On-site Inspection. In epicentral regions of the underground nuclear explosions with depths down to 100–120 m sites with lower velocity values were noted down to 3,0–3,5 km/s for P and 1,6–1,8 km/s for S waves; these were interpreted as slabbing occurrences. The rocks within the slabbing zones are different with increased Poisson’s ratio values up to 0.35, which is 0.07 higher than its pre-explosion values. Using pre- and post-explosion parameters of the background medium (seismic waves velocity, mountain rocks density, elastic modules) a parameter of fractures’ density was calculated within industrially changed layer. A correlation of post-explosion fracture of mountain rocks with engineering and technical parameters of underground explosions was established – a weight of nuclear charge and its laying depth.

Integral Equation Methods With Multiple Scattering and Gaussian Beams in Inhomogeneous Background Media for Solving Nonlinear Inverse Scattering Problems

I develop inverse scattering methods for velocity reconstruction in the subsurface, based on multiple scattering theory, Gaussian beams and nonlinear Born approximation. This method is based on a new version of the distorted Born iterative inverse scattering method. It directly uses an explicit representation of the data sensitivity function in terms of Green functions, rather than the indirect optimization approach based on the adjoint state method. I propose three direct scattering methods for forward modeling, namely, Gaussian beam integral equation propagators, nonlinear Born approximation, and multiple scattering. First, I apply a sensitivity kernel incorporating the Gaussian beam summation-based integral equation method and compute the background medium Green's function in an inhomogeneous medium. Then I extend an approximate solution of the Lippmann-Schwinger equation, referred to as nonlinear Born approximation to inverse scattering problem. I apply the nonlinear Born approximation to forward modeling at each iteration. Furthermore, I combine the Gaussian beams with the multiple scattering theory for nonlinear scattering problems. I obtain the inversion results using the proposed approaches, which shows the capability for inverse scattering imaging. I have carried out several numerical experiments that involve reconstructing the resampled Marmousi and Society of Exploration Geophysicists (SEG)/European Association of Geoscientists and Engineers (EAGE) salt models from its smoothed version.

SLIM: A Well-Conditioned Single-Source Boundary Element Method for Modeling Lossy Conductors in Layered Media

The boundary element method (BEM) enables the efficient electromagnetic modelling of lossy conductors with a surface-based discretization. Existing BEM techniques for conductor modelling require either expensive dual basis functions or the use of both single- and double-layer potential operators to obtain a well-conditioned system matrix. The associated computational cost is particularly significant when conductors are embedded in stratified media, and the expensive multilayer Green's function (MGF) must be invoked. In this work, a novel single-source BEM formulation is proposed, which leads to a well-conditioned system matrix without the need for dual basis functions. The proposed single-layer impedance matrix (SLIM) formulation does not require the double-layer potential to model the background medium, which reduces the cost associated with the MGF. The accuracy and efficiency of the proposed method is demonstrated through realistic examples drawn from different applications.

Resolving small-scale cold circumgalactic gas in TNG50

ABSTRACT We use the high-resolution TNG50 cosmological magnetohydrodynamical simulation to explore the properties and origin of cold circumgalactic medium (CGM) gas around massive galaxies (M⋆ > 1011 M⊙ ) at intermediate redshift ($z \sim 0.5$). We discover a significant abundance of small-scale, cold gas structure in the CGM of ‘red and dead’ elliptical systems, as traced by neutral H i and Mg ii. Halos can host tens of thousands of discrete absorbing cloudlets, with sizes of order a kpc or smaller. With a Lagrangian tracer analysis, we show that cold clouds form due to strong $\delta \rho / \bar{\rho } \gg 1$ gas density perturbations that stimulate thermal instability. These local overdensities trigger rapid cooling from the hot virialized background medium at ∼107 K to radiatively inefficient ∼104 K clouds, which act as cosmologically long-lived, ‘stimulated cooling’ seeds in a regime where the global halo does not satisfy the classic tcool/tff < 10 criterion. Furthermore, these small clouds are dominated by magnetic rather than thermal pressure, with plasma β ≪ 1, suggesting that magnetic fields may play an important role. The number and total mass of cold clouds both increase with resolution, and the mgas ≃ 8 × 104 M⊙ cell mass of TNG50 enables the ∼ few hundred pc, small-scale CGM structure we observe to form. Finally, we make a preliminary comparison against observations from the COS-LRG, LRG-RDR, COS-Halos, and SDSS LRG surveys. We broadly find that our recent, high-resolution cosmological simulations produce sufficiently high covering fractions of extended, cold gas as observed to surround massive galaxies.

On the solution of direct and inverse multiple scattering problems for mixed sound-soft, sound-hard and penetrable objects

In this work we consider a scattering problem governed by the two-dimensional Helmholtz equation, where some objects of different nature (sound-hard, sound-soft and penetrable) are present in the background medium. First we propose and analyze a system of boundary integral equations to solve the direct problem. After that, we propose a numerical method based on the computation of a multifrequency topological energy based imaging functional to find the shape of the objects (without knowing their nature) from measurements of the total field at a set of observation points. Numerical examples show that the proposed indicator function is able to detect objects of different nature and/or shape and size when processing noisy data for a rich enough range of frequencies.

EnKF-levelset method for an acoustics inverse medium scattering problem

This paper demonstrates a numerical method for reconstructing penetrable obstacles in a homogeneous background medium. The ensemble Kalman method is proposed as an inversion solver, and the level set technique is discussed as a flexible way to tracking the boundary. Through some numerical examples, we show that the proposed method is effective and flexible even without the priori number knowledge of the scatterers.

Selective Emitter with Engineered Anisotropic Radiation to Minimize Dual-Band Thermal Signature for Infrared Stealth Technology.

Selective emitters comprising plasmonic resonators have been exploited for cooling devices or infrared stealth technology. While selective emitters have been designed using odd-order resonances, even-order resonances also emit anisotropic thermal radiation signals. Thermal radiation by even-order resonances in selective emitters can be experimentally detected by thermal imaging cameras, and such thermal emissions often degrade the observability of infrared detectors, rendering them inapplicable to infrared stealth technology. Here, a selective emitter with extremely low thermal radiation signature in a dual-band range, a detection range by an infrared detector, is proposed with engineering anisotropic thermal radiation by even-order resonances. To minimize infrared signature in a dual-band range, the characterization of even-order resonances of gap plasmon metasurfaces is achieved based on vectorial diffraction within a relative error of 10%. Thermal radiation by even-order resonance has been shown to be highly directional and can be experimentally measured using mid-wave infrared images. Based on model prediction, the proposed selective emitter reduces mid-wave infrared signatures and long-wave infrared signatures by factors of 37.95 and 38.06, respectively, compared with those of blackbody surfaces. In addition, numerically confirmed thermal signature reduction and captured mid-wave infrared images indicate excellent thermal camouflage performance of the selective emitter with background medium. Thus, the characterization of even-order resonances provides a basis for the design of metasurfaces that can be employed for multispectral applications, especially infrared stealth technology.

Fluctuation-dissipation theorem and fundamental photon commutation relations in lossy nanostructures using quasinormal modes

We provide theory and formal insight on the Green function quantization method for absorptive and dispersive spatial-inhomogeneous media in the context of dielectric media. We show that a fundamental Green function identity, which appears, e.g., in the fundamental commutation relation of the electromagnetic fields, is also valid in the limit of non-absorbing media. We also demonstrate how the zero-point field fluctuations yields a non-vanishing surface term in configurations without absorption, when using a more formal procedure of the Green function quantization method. We then apply the presented method to a recently developed theory of photon quantization using quasinormal modes [Franke et al., Phys. Rev. Lett. 122, 213901 (2019)] for finite nanostructures embedded in a lossless background medium. We discuss the strict dielectric limit of the commutation relations of the quasinormal mode operators and present different methods to obtain them, connected to the radiative loss for non-absorptive but open resonators. We show exemplary calculations of a fully three-dimensional photonic crystal beam cavity, including the lossless limit, which supports a single quasinormal mode and discuss the limits of the commutation relation for vanishing damping (no material loss and no radiative loss).

Effective Resonant Model and Simulations in the Time-Domain of Wave Scattering from a Periodic Row of Highly-Contrasted Inclusions

The time-domain propagation of scalar waves across a periodic row of inclusions is considered in 2D. As the typical wavelength within the background medium is assumed to be much larger than the spacing between inclusions and the row width, the physical configuration considered is in the low-frequency homogenization regime. Furthermore, a high contrast between one of the constitutive moduli of the inclusions and of the background medium is also assumed. So the wavelength within the inclusions is of the order of their typical size, which can further induce local resonances within the microstructure. In Pham et al. (J. Mech. Phys. Solids 106:80–94, 2017), two-scale homogenization techniques and matched-asymptotic expansions have been employed to derive, in the harmonic regime, effective jump conditions on an equivalent interface. This homogenized model is frequency-dependent due to the resonant behavior of the inclusions. In this context, the present article aims at investigating, directly in the time-domain, the scattering of waves by such a periodic row of resonant scatterers. Its effective behavior is first derived in the time-domain and some energy properties of the resulting homogenized model are analyzed. Time-domain numerical simulations are then performed to illustrate the main features of the effective interface model obtained and to assess its relevance in comparison with full-field simulations.

Tidal forces from the wake of dynamical friction: warps, lopsidedness, and kinematic misalignment

ABSTRACT A galaxy moving through a background of dark matter particles induces an overdensity of these particles or a wake behind it. The back reaction of this wake on the galaxy is a force field that can be decomposed into an effective deceleration (called dynamical friction) and a tidal field. In this paper, we determine the tidal forces, thus generated on the galaxy, and the resulting observables, which are shown to be warps, lopsidedness, and/or kinematic-photometric position angle misalignments. We estimate the magnitude of the tidal-like effects needed to reproduce the observed warp and lopsidedness on the isolated galaxy IC 2487. Within a realistic range of dark matter distribution properties, the observed, warped, and lopsided kinematical properties of IC 2487 are possible to reproduce (the background medium of dark matter particles has a velocity dispersion of $\lesssim 80\, {\rm km\, s^{-1}}$ and the density $10^4{\!-\!}10^5\, {\rm M_\odot \, kpc^{-3}}$, more likely at the lower end). We conclude that the proposed mechanism can generate warps, lopsidedness, and misalignments observed in isolated galaxies or galaxies in loose groups. The method can be used also to constrain dark matter spatial and velocity distribution properties.